The present invention relates to a calender roll for use in a super calender for magnetic tapes and, particularly, to a elastic calender roll of a super calender which is to be used in calendering the magnetic surface of a magnetic recording medium such as video tape, memory tape, audio tape, etc.
There are known four kinds of calendering methods for magnetic tape. According to a first method, the calendering effect is obtained by suitably selecting a dispersion agent of a magnetic painting liquid at the preparation stage of the magnetic painting liquid, and a dispersion method thereof onto a tape base. It is known in this method, however, that the electromagnetic conversion characteristics of the final tape, particularly the sensitivity and S/N ratio thereof, are insufficient. In the second method, the calendering is performed by rubbing the surfaces of the magnetic layers of tapes with each other at a high relative velocity. The quality of the tape provided by the second method is relatively low due to a considerable removal of the magnetic particles from the tape during the rubbing. According to the third method, the magnetic layer surface of the tape is polished by fur or plastic or metal brushes. The tape provided by the third method has, however, a relatively rough surface which is not suitable to use for high density recording.
The fourth method is to use a super calender using a pressure roll. The pressure roll, i.e. an elastic roll is usually made by raw cotton, absorbent cotton, pulp, refined cotton or wool. The principle of the super calender and problems inherent to the elastic roll are disclosed in Japanese Patent Application Laid-Open No. 51-103404 (U.S. Ser. No. 657,800 filed on Feb. 13, 1976, now U.S. Pat. No. 4,128,673). As described in the Laid-Open No. 51-103404, the tape calendered by the roll made of cottons, woolen paper etc. is not suitable to use for present video tapes because the roll can not provide a sufficiently smooth surface due to the roll's fibrous surface pattern which is stamped on the tape surface as a result of the calendering.
Japanese Patent Application Laid-Open No. 51-103404 discloses a roll made of polyamide resin. This roll can provide a very smooth or flat magnetic tape surface. However, since the softening point of polyamide resin is relatively low, it is impossible to greatly increase the calendering pressure i.e., nipping pressure, and/or the rotation speed of the roll without deformation of the roll. This causes a low producibility of tape. That is, polyamide resin, such as nylon, has a large thermal expansion coefficient and low thermal conductivity. Therefore, a calender roll made of such a material is heated by self-heating due to the pressurized rotation thereof and expands, thereby producing a calendered tape of low quality.
A roll made of rubber which can also provide a smooth surface is difficult to manufacture due to the difficulty of balancing the surface smoothness and hardness thereof.